1. Field of the Invention
This invention relates broadly to a medical device used in refractive surgery treatment to alter the curvature of the cornea. More specifically, this invention relates to a heating device which provides particular advantage in the treatment of hyperopia and astigmatism.
2. State of the Art
Refractive surgery is used to alter the curvature of the cornea to correct vision. Many techniques are currently employed to alter corneal curvature.
With respect to myopia, or "near-sightedness", the treatments of the art primarily aim to "flatten" the cornea; i.e., increase the radius of curvature of the cornea. For example, in radial keratotomy, a knife is used to make near full thickness incisions into the cornea to increase the radius of corneal curvature. The currently preferred tool of choice in the treatment of myopia is the excimer laser, which is used to selectively ablate corneal tissue and alter the corneal shape. In addition, other techniques using radio frequency waves or a holmium laser have also been used, whereby the corneal stroma is heated to induce an increased curvature of the cornea.
However, a convenient, effective, and safe method for treating hyperopia, or "far-sightedness", in which it is desired to decrease the corneal radius of curvature, has remained elusive. Refractive surgery for hyperopia has centered on the principal of inducing shrinkage of corneal tissues to "steepen" the central cornea. For instance, the holmium laser is used to heat the corneal stroma at several points around the visual axis. Upon heating, the stromal collagen (that is, the supporting tissue of the cornea) at these locations is induced to shrink and thereby reduce the radius of curvature of the central cornea. A similar approach has been advocated with the use of radio frequency waves, whereby electrodes are inserted into the corneal stroma and heated via current transmission or direct radio frequency waves to heat surrounding corneal tissues. As with the holmium laser techniques, the effect is to shrink collagen around the visual axis, and induce a steepening of the central cornea.
One of the main problems with these techniques is regression. After heating, the stromal collagen regresses toward its original conformation and shape and the hyperopia returns. Another problem is that uneven stromal collagen shrinkage at the heating points around the visual axis may result in irregular astigmatism, thereby exacerbating the visual deficiency.
Moreover, the excimer laser has not proved completely successful in treating hyperopia, primarily due to resulting stromal haze and regression.